Conventional radiation imaging systems may utilize photoconductive materials to absorb incident radiation representative of an object. U.S. Pat. No. 4,176,275 discloses a digital x-ray imaging system in which a radiation source is positioned to direct a radiation image of an object onto the upper surface of a detector plate. The detector plate includes a suitable photoconductive material that absorbs the radiation and produces electron-hole pairs (first charge carriers) which may be separated from each other by an electric field applied across the photoconductor, creating a latent image of the object at the surface of the photoconductor which is typically a thin planar layer within the detector plate. A narrow beam of scanning radiation substantially completes discharge of the photoconductor by creating the motion of a second set of charge carders. The distribution of these second charge carders in the plane of the photoconductor is affected by the distribution of the first charge carriers, i.e., by the latent image. The motion of the second charge carriers is detected and digitized in an appropriate circuit, thereby capturing the latent image in digital form.
The detector plate is a multi-layered device having a plane parallel stack of first conductive, dielectric (insulative), photoconductive and second conductive layers. The first conductive layer provides the surface to which the radiation image is directed, and therefore both the first conductive layer and the dielectric layer must be substantially transparent to the radiation energy produced by the radiation source so that it can reach the photoconductive layer. A D.C. voltage source is connected between the first and second conductive layers, with the polarity typically being that the first conductive layer is positive with respect to the second conductive layer.
During use, large voltages of up to 10 kilovolts are applied across the sandwich structure of the detector plate, resulting in electric fields as high as 10 v/micron across the dielectric. Under the application of this high voltage and repeated use of the detector plate, the first conductive layer tends to fail electrically due to (1) cracking, and/or (2) arcing from the first conductive layer to ground which is typically the second conductive layer of the detecter plate or possibly the cassette in which the detector plate is housed. This type of arcing not only breaks down the first conductive layer but also could potentially damage the rest of the detector plate. Cracking typically occurs after repeated application of the high voltage, and can be considered as a surface "brush discharge" whereby the first conductive layer is ablated in the discharge area leaving the dielectric layer exposed.